Rotaxanes are mechanically interlocked molecular architectures consisting of a dumbbell-shaped molecule, the “axle,” that threads through a ring called a macrocycle. Because the rings can spin around and slide along the axle, rotaxanes are promising components of molecular machines. While most rotaxanes have been entirely organic, the physical properties desirable in molecular machines are mostly found in inorganic compounds. Working together, two British groups at the University of Edinburgh and the University of Manchester have bridged this gap with hybrid rotaxanes, in which inorganic rings encircle the organic axles. The hybrid architecture greatly increases their range of useful physical properties, such as the magnetism based on molecular magnets that may make them suitable as qubits for quantum computers.

Chemistry Aids Quantum Computing

Quantum bits or qubits are the fundamental information carrying structures in quantum computing. Unlike the 0 and 1 binary bits that serve this purpose in conventional computers, qubits make use of quantum properties to store information in a much more complex way that potentially speeds up some kinds of computation and makes it more secure. While researchers have demonstrated simple quantum logic circuits, the search for structures that could be expected to function in the larger assemblies needed for practical computing continues at a rapid pace with significant government funding at laboratories around the world.

Two British groups have combined forces to devise a way to array qubits based on the magnetic properties of molecules (molecular magnets) in nanostructures so that the qubits can interact when necessary and be isolated when not, while not being perturbed by the rest of the structure. Their solution draws on supramolecular chemistry, a field that focuses on merging previously assembled molecular subunits into a larger structure that performs the desired function. In particular, they constructed structures with one or more rings containing magnetically active metals (the qubits) around an axel or thread with “stoppers” on the end to keep the rings from sliding off, a structure known as a rotaxane. Controlling the rotation of the rings around the thread and the shuttling motion along the ring, operations needed for quantum computing, are yet to come.

While conventional computers store information in binary bits, quantum computers would use a more complex equivalent—a qubit—which not only represents 0 and 1, but all possible superpositions of the quantum states representing 0 and 1 simultaneously. The complexity introduces the possibility of some calculations being performed much more quickly than could be achieved with conventional computers. Previous work by the group at Manchester had looked at molecular magnets as possible qubits, but how to create suitable nanostructures in which to place them remained.

The “bottom-up” approach to fabrication of nanostructures requires chemical methods to link functional building blocks into larger structures. A problem arises when the electronic structure of the building block is to be retained in the material constructed. Traditional chemistry would link blocks through covalent bonds, but these inevitably involve a strong interaction between the electronic structure of one block and of its neighbor, and hence the individual character of building blocks could be lost. Fortunately, the Edinburgh group had shown it could make very complex interlocked structures using organic supramolecular chemistry, which focuses on chemical systems made up of a discrete number of assembled molecular subunits or components. The supramolecular chemistry is such that many related structures can be made from similar basic building blocks.

The synthesis of hybrid organic-inorganic rotaxanes, with the magnetically active metal sites within the inorganic ring shown as filled polyhedra. The orange polyhedra represent chromium(III), purple polyhedra cobalt(II), and blue polyhedra copper(II). The three rotaxanes shown are a [2]rotaxane 4a (one ring on one thread), which behaves as a molecular shuttle, a [3]rotaxane 5b (two rings on one thread) and a [4]rotaxane 6b (two rings on two threads). The numbers in brackets represent the total number of rings and threads in one structure.

Bringing together the work of these two groups has led to interlocked assemblies where the potential qubits are brought into close proximity without strong interactions between the electronic structures of the qubits. The structures formed are rotaxanes that feature the inorganic molecular magnet acting as the ring about an organic axle, with bulky stoppers attached to the end of the axle to prevent the ring sliding off the end. The combination of organic and inorganic chemistry has allowed synthesis of [2]-, [3]- and [4]rotaxanes in good yields. The [4]rotaxane, in which two threads pass through two rings, has only a single precedent in the literature.

X-ray single crystal diffraction at ALS Beamline 11.3.1 was used to verify the structures of the [4]rotaxane produced and of several related compounds. The next stage was to show molecular motion by NMR spectroscopy. Rotation of the ring about the axle is very fast, but motion of the ring along the axle in the [2]rotaxane (a molecular shuttle) occurs about once per second. The very different time-scales for the two types of motion is unusual.

The structures of the [3]- and [4]rotaxanes as determined by x-ray single crystal diffraction. The organic threads are shown as space filling spheres (hydrogen, white; carbon, grey; oxygen, red; and nitrogen, blue) and the inorganic rings as a mixture of polyhedra (chromium, orange; cobalt, purple; and copper, blue) and stick representations (carbon, blue; oxygen, red; and fluorine, green).

Future steps in the project are to introduce methods for switching interactions on and off between the qubits on the axle and to look for means for controlling the speed of molecular shuttling. The threaded architecture ensures that the electronic, magnetic, and paramagnetic characteristics of the inorganic rings could be influenced by the organic portion of the rotaxane. A photo-active organic component could allow use of light as a means to switch on and off the interactions between qubits threaded onto a single axle during computation. It is also possible to imagine much more complex interlocked structures through further modifications of the chemistry.

Research funding: the Engineering and Physical Sciences Research Council (UK), the European Commission Network of Excellence “MAGMANet,” and The Royal Society (UK). Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.